The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.
Thermoregulation in humans includes a temperature range for a core temperature, where the core comprises internal organs (including the brain), and a specific temperature range for a shell, where the shell comprises skin and extremeties (hands, legs, nose, ears, etc.).
In humans, the hypothalamus regulates the core temperature within a narrow range, approximately 36.1° C. to 37.8° C. (96.98° F. to 100.04° F.). In other words, the hypothalamus functions as a thermostat preset at 37° C., and maintains this core temperature range by constantly regulating production and loss of heat.
Body heat is generated by the catabolism of intracellular proteins, carbohydrates, and fats provided by our nutrition. The body heat can be further increased by increased muscle activity, such as during exercise.
The daily excess heat production is eliminated along a gradient of temperature between core (37° C.), the skin (33° C.) and environment if it is less than 33° C. The heat is transported by the blood to the skin and then dissipated to the environment. The hypothalamus adjusts the heat loss by regulating the skin and the core circulation. For example, if there is excess heat production, the hypothalamus closes (vasoconstriction) the core circulation and opens (vasodilate) the skin circulation, which shifts up to 70% of our cardiac output away from the core organs (except heart and brain). It also increases the cardiac output from 5 liters per min to up to 20 liters per min when exercising in high ambient temperature.
Conversely, in a situation with a low environmental temperature, the skin temperature drops and thermoreceptors on the skin send signals to the hypothalamus, which immediately reacts by insulating the body via closure of the skin circulation. Our extremities (fingers, nose, ears, etc.) may become blue and painful. If this action is not sufficient to maintain the core temperature at 37° C., the hypothalamus triggers muscle shivering which produces heat.
Exposure to a hot environment may result in heatstroke, which is a life-threatening condition characterized by a rapid increase in core temperature, multiple organ dysfunction, and tissue injury. It is a leading cause of mortality and neurological damage when there is an unaccustomed and sustained increase in climatic temperature, such as during a heat wave, and/or any prolonged exposure without proper precautions. Hyperthermia is the primary mechanism of cell death and tissue injury in heatstroke. The severity of injury is a function of the degree of hyperthermia and duration of exposure. An objective in the treatment of hyperthermia is to decrease body temperature as quickly as possible to prevent irreversible damage and death. Cooling techniques can induce severe shivering and skin vasoconstriction, which can increase heat production and decrease heat elimination resulting in the opposite of the desired cooling effect. Also, the rate of cooling can be unpredictable, varying from failure to cool to excessive cooling.